The flush toilet is an ancient sanitation invention, with evidence of its use found even in Neolithic times. Flush toilets use gravity to refresh the water in the toilet bowl upon flushing. Typically, a toilet tank comprises a tank (or cistern), a bowl, and a handle. An inlet valve within the tank controls the water supply coming into the tank. If the tank is empty, then the inlet valve is open, allowing water to flow in. A float ball attached to the inlet valve by a float rod rises as water flows into the tank. When the tank is full, the float rod is pressing against the inlet valve hard enough to stop the flow of water from the inlet valve.
In typical household embodiments, the handle is attached by a chain to a piston (or flapper) controlling flow of water from the tank to the bowl. The piston plugs a hole leading from the tank to the bowl. When a user pulls the handle, the chain pulls up on the piston, forcing water through a siphon into the bowl. The piston is usually covered by a plastic membrane, which is constructed to be sucked out of the way by the emptying siphon tube water, allowing the rest of the water in the tank to rush past the piston, and over the top of the siphon, emptying the rest of the tank. The piston membrane then drops back onto the piston, ready for the next flush operation, and the chain usually falls to rest near the piston. See generally Wikijunior: How Things Work/Flush Toilet, WIKIBOOKS, https://en.wikibooks.org/wiki/Wikijunior: How_Things_Work/Flush_Toilet (last visited Mar. 30, 2018).
However, occasionally the chain catches on the float lever or on or under the piston. As a result, the piston does not fully reseal, and water can freely move (albeit at a slower flow rate) from the bowl to the tank. This is one of the main causes of toilets “running.”